Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Such fuel cells generally employ a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
At the anode, the fuel permeates the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to complete the electrochemical reaction and form a reaction product.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to complete the electrochemical reaction and form liquid water as the reaction product.
In conventional fuel cells, the MEA is interposed between two fluid-impermeable, electrically conductive plates, commonly referred to as the anode and the cathode plates, respectively. The plates serve as current collectors, provide structural support for the porous, electrically conductive electrodes, provide means for carrying the fuel and oxidant to the anode and cathode, respectively, and provide means for removing water formed during operation of the fuel cell. When the channels are formed in the anode and cathode plates, the plates are referred to as fluid flow field plates. When the anode and cathode plates overlay channels formed in the anode and cathode porous material, the plates are referred to as separator plates.
Reactant feed manifolds are generally formed in the anode and cathode plates, as well as in the MEA, to direct the fuel (typically substantially pure hydrogen or hydrogen-containing reformate from the conversion of hydrocarbons such as methanol or natural gas) to the anode and the oxidant (typically substantially pure oxygen or oxygen-containing gas) to the cathode via the channels formed in either the fluid flow field plates or the electrodes themselves. Exhaust manifolds are also generally formed in the anode and cathode plates, as well as the MEA, to direct unreacted fuel and oxidant, as well as water accumulated at the cathode, from the fuel cell.
Multiple fuel cell assemblies comprising two or more anode plate/MEA/cathode plate combinations, referred to as a fuel cell stack, can be connected together in series (or in parallel) to increase the overall power output as required. In such stack arrangements, the cells are most often connected in series, wherein one side of a given fluid flow field or separator plate is the anode plate for one cell, the other side of the plate is the cathode plate for the adjacent cell, and so on.
Perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its Nafion trade designation, have been used effectively in electrochemical fuel cells. Fuel cells employing Nafion-type cation exchange membranes require accumulated water to be removed from the cathode (oxidant) side, both as a result of the water transported across the membrane with cations and product water formed at the cathode from the electrochemical reaction of hydrogen cations with oxygen. A new type of experimental perfluorosulfonic ion exchange membrane, sold by Dow under the trade designation XUS 13204.10, appears to have significantly less water transported with hydrogen cations across the membrane. Fuel cells employing the Dow experimental membrane thus tend to accumulate less on the cathode (oxidant) side, as the accumulated water at the cathode is essentially limited to product water formed from the electrochemical reaction of hydrogen and oxygen.
A typical prior art fluid flow field plate, exemplified by General Electric Company and Hamilton Standard in a 1984 report for the U.S. Department of Energy (LANL No. 9-X53-D6272-1), included a plurality of parallel open-faced fluid flow channels formed in a major surface of a rigid, electrically conductive plate. The parallel channels extended between an inlet header and an outlet header formed in the plate. The parallel channels were typically rectangular in cross-section, and about 0.03 inches deep and about 0.03 inches wide. The inlet header was connected to an opening in the plate through which a pressurized reactant (fuel or oxidant) stream is supplied. The outlet header was connected to an opening in the plate through which the exhaust stream is discharged from the cell. In operation, the reactant stream ran from the inlet to the inlet header and then to the parallel channels from which reactant from the stream diffused through the porous electrode material to the electrocatalytically active region of the MEA. The stream then flowed to the outlet header and then to the outlet from which it was exhausted from the fuel cell.
Watkins U.S. Pat. Nos. 4,988,583 and 5,108,849 issued Jan. 29, 1991 and Apr. 28, 1992, respectively, describe fluid flow field plates which include a fluid supply opening and a fluid exhaust opening formed in the plate surface. Continuous open-faced fluid flow channels formed in the surface of the plate traverse the central area of the plate surface in a plurality of passes, that is, in a serpentine manner. Each channel has a fluid inlet at one end and a fluid outlet at the other end. The fluid inlet and outlet of each channel are directly connected to the fluid supply opening and fluid exhaust opening, respectively. The continuous channel design promotes the forced movement of water through each channel before the water can coalesce, thereby promoting uniform reactant flow across the surface of the cathode.
U.S. patent application Ser. No. 07/975,791 filed Nov. 13, 1992, incorporated by reference herein in its entirety, describes and claims a fluid flow field plate for electrochemical fuel cells in which the inlet and outlet flow channels are discontinuous. The employment of discontinuous flow channels, as described in U.S. patent application Ser. No. 07/975,791, has several advantages:
1. Improved performance, particularly at higher reactant inlet pressures, resulting from (a) the increased partial pressure of reactant gases at the active front (less reactant depletion in the stream compared to the continuous, serpentine channels in which the reactant is continuously depleted as the stream flows away from the inlet), (b) more effective water removal due to better access of the reactant stream to the electrocatalytically active region at the membrane/electrode interface, (c) more uniform current density due to more even distribution of the reactant stream across the electrocatalytically active area of the fuel cell and the avoidance of water pooling in the flow channels, and (d) lower flow field plate/electrode contact resistance due to the use of a decreased amount of the flow field plate surface to accommodate the flow channels.
2. Improved fuel cell lifetime resulting from (a) the ability to reduce the compressive load on the electrodes due to decreased contact resistance between the flow field plates and the electrodes, and (b) more uniform reactant gas relative humidity due to the reduced depletion of reactant in the stream compared to the continuous, serpentine flow channel design.
3. Reduced manufacturing costs resulting from (a) the ability to reduce the amount of graphite plate milling required for continuous channels and to relax the tolerances required for the channel dimensions, (b) a wider range of materials and fabrication techniques permitted with the discontinuous flow channel design, such as stamping of flow field stencils, to be employed, particularly the use of thinner electrically conductive sheet materials, as the discontinuous channels do not require the thickness and rigidity of the electrically conductive plates in which continuous, serpentine flow channels are formed, and (3) the ability to employ a stenciled graphite foil laminate, thereby reducing the weight (and cost) associated with rigid graphite flow field plates.
Conventional methods of fabricating fluid flow field plates require the engraving or milling of flow channels into the surface of the rigid graphite plates. The resin-impregnated graphite plates tend to have a high impurity content, relatively poor fracture resistance, and significant fluid (especially gas) permeability, particularly in the case of thinner rigid graphite plates, and most particularly in the case of plates less than 0.125 inches thick. Conventional fluid flow field plates are also expensive, both in raw material costs and in machining costs. The machining of channels and the like into the graphite plate surfaces causes significant tool wear and requires significant processing times.
Accordingly, it is an object of the invention to provide an improved, laminated fluid flow field assembly for use in electrochemical fuel cells that is reduced in weight and volume, and that is simpler and less expensive to manufacture than conventional fluid flow field plates
Another object of the invention is to provide an improved electrochemical fuel cell that includes a laminated fluid flow field plate having improved weight, volume and manufacturability characteristics.
A further object of the invention is to provide an improved method of fabricating a laminated fluid flow field plate for use in electrochemical fuel cells.